1. Field of the Invention
The present invention relates to a terminal for transmitting data over a bidirectional analog channel, simultaneously in both directions, including:
a hybrid coupler connected to the bidirectional channel and to two unidirectional analog channels, one for emitting and the other for receiving,
means, connected to the emitting channel, for generating at least one analog signal from local data to be emitted towards another terminal,
means, connected to the receiving channel, for estimating, from a analog received signal, distant digital data coming from the other terminal, coded by means of symbols, at a modulation rate expressed in bauds, non synchronous with said means for generating said emitted signal, and
at least one echo canceller disposed between said means for generating said emitted signal, and said means for estimating the distant data, adapted so as to estimate the echo signal and to subtract an estimated echo signal from said received signal.
With terminals of this type, a bidirectional channel, for example a simple twin wire connection, may be used for the transmission ("full duplex") of data between two terminals, simultaneously in both directions. In fact, because of the echo canceller, the main drawback of this type of connection, namely the presence in the receiving channel of an echo signal resulting from the parasite reflections of the locally emitted signal, and capable of disturbing the reception, is eliminated.
2. Description of the Prior Art
Terminals of the above defined type are already known, in particular from the article by S. B. Weinstein "A Passband Data-Driven Echo Canceller for Full-Duplex Transmission on Two-Wire Circuits", IEEE Trans. on Com. Vol. Com-25 N.sup.0 7, July 1977, pages 654-665.
In a terminal of this type, the data to be emitted is digital data coded by means of symbols having a certain modulation rate, expressed in bauds, further called emission baud frequency. The echo canceller has at least one digital filter which calculates the estimated echo signal directly from the symbols to be emitted. Adjustment of the coefficient of the digital filter is made, for example, during an initialisation procedure, during which, with the distant terminal not emitting the nearby terminal emits a signal. It is then certain that the signal received is equal to the echo signal and the coefficients of the filter of the canceller are then adjusted so as to cancel out the difference between the true echo signal and the estimated echo signal. When the causes of the echo are not likely to vary in time, the terminal may then operate normally. When the causes of the echo are likely to change during time, the terminal is caused to operate by permanently adapting the coefficients of the filter of the canceller so that the correlation function between, on the one hand, the difference between the signal received and the estimated echo signal and, on the other hand, the signal at the input of the canceller, is zero. In fact, the preceding difference represents the sum of the signal from the distant terminal and an echo residue, and the correlation function between the signal from the distant terminal and the signal at the input of the canceller is zero. Thus, the echo residue can be cancelled out adaptively even in the presence of a signal from the distant terminal.
Because the echo canceller operates from the signals to be emitted, it is synchronized on the emission baud frequency. Since this later is not synchronous with the reception baud frequency, that is to say the modulation rate of the symbols transmitted by the distant terminal, the signal received is sampled at a sampling frequency chosen equal to a multiple of the emission baud frequency. Thus, the frequency for sampling the received signal and the work rate of the echo canceller are synchronized, which makes it possible for this latter to function without the information contained in the signal received being lost. For, naturally, the frequency which is a multiple of the emission baud frequency which serves for sampling the received signal is chosen fairly large so that Shannon's theorem is always respected, considering the expected spectral characteristics of the received signal, and in particular of the echo signal.
However, since the means for estimating the distant data are digital means working on samples, these samples must be obtained by sampling at a frequency which is a multiple of the reception baud frequency, or at a frequency equal to this baud frequency, which means that the samples of the difference between the received signal and the estimated echo signal must be passed through an analog interpolation filter, so as to reform an analog signal which may be sampled at the multiple frequency of the reception baud frequency, or at this baud frequency, before reception processing. Naturally, in order to reduce the number of operations to be carried out for echo cancellation, it is preferably to choose the sampling frequency, a multiple of the emission baud frequency, so that it is the lowest frequency possible. However, the lower frequency, the greater the constraints concerning the steepness of the slopes of the interpolation filter, and a compromise must in general be found.
Furthermore, in a digital construction of such a terminal, using for example processors, processing in the echo canceller and processing in the distant data estimation means, that is to say in the receiver, each required relatively high computing powers. Since these two processing operations are carried out at non synchronized timing, it is necessary to control the sharing of the processing time between the echo canceller and the receiver, which complicates the terminal.
Finally, in a terminal of the above type, if the signal propagating over the bidirectional channel undergoes appreciable and possibly variable distortion in time, it is possible, as is known, to compensate such distortion by means of a filter, called "equalizing filter" acting on the received signal, possibly adaptive so as to follow the variations in time of the characteristics of the bidirectional transmission channel. As is known, such a filter is for example a digital filter, whose coefficients are adjusted so as to compensate for the distortions of the transmission channel. Adjustment of the coefficients is made, for example, during an initialization procedure during which the distant terminal emits a predetermined sequence of signals known by the nearby terminal. This letter is adapted for comparing the sequence of estimated distant symbols with a predetermined sequence and for adjusting the coefficients of the equalizing filter so as to provide equality between these two sequences. When the characteristics of the transmission channel are not likely to change during time, the terminal can thus function normally. When the characteristics of the transmission channel are likely to change in time, the terminal is operated by permanently adapting the coefficients of the equalizing filter, using as criterion the fact that, since the coding signals can only take on a certain number of distinct and known states, the difference between the value of the signal received after equalization and echo cancellation and the value of the closest known state must be zero if the equalizing filter suitably compensates for the distortions in the transmission channel.
However, in a terminal of the above type, and in the cases where the echo canceller, like the equalizer, are adaptive, because their operating rates are different, their adaptations are controlled by different signals. It follows, that, in the frequent case where the equalizer is disposed downstream of the point where the echo canceller subtracts the estimated echo, the signal which controls the echo canceller includes noise due to the signal coming from the distant terminal, whereas the input of the equalizer includes noise due to the echo residue. The result is unsatisfactory performances of the whole of the terminal.